B cells in epithelial and perivascular compartments of human adult thymus.

The thymus is the site of T-cell differentiation. However, the relatively recent observation that B cells are also present in the human thymus has prompted studies to determine the origin and function of these B cells. Our studies show that phenotypically distinguishable B cell populations are located within both the thymic medulla and the thymic perivascular space and that cellular trafficking occurs between these compartments, including B cells trafficking from the periphery. The numbers of thymic B cells increase with age, correlating with increases in lymphocyte-rich regions of thymic perivascular space that are prominent between ages 10 and 50 years. B cells within both thymic epithelial and perivascular compartments contain mutated immunoglobulin VH sequences characteristic of post-germinal center B cells, suggesting that the B cells that most often give rise to thymic B-cell lymphomas may originate from either the thymic medulla or perivascular space.